The invention relates to films, dispersions and optoelectronic devices comprising conducting polymer.
Optoelectronic devices are devices characterized by the interconversion of light and electricity. Optoelectronic devices either produce light or use light in their operation. Examples of optoelectronic devices include electroluminescent assemblies (e.g. light emitting diodes), laser diode and photovoltaic assemblies (e.g. photodiodes, photodetector and solar cells).
Electroluminescence is nonthermal conversion of electrical energy into light. An electroluminescent (“EL”) assembly is characterized by the emission of light and the flow of electric current when an electric potential (or voltage) is applied. Such assemblies include light emitting diodes (“LEDs”), which are injection type devices. Organic LEDs (OLEDs) comprise organic semiconductors, such as conjugated low molecular weight molecules (small molecules) and high molecular weight polymers.
Organic semiconductors, especially conjugated polymers, combine the optical and electrical properties of inorganic semiconductors and the mechanical strength, such as flexibility, of plastics. Therefore, OLEDs have many advantages over other competing technologies and can be used in many different applications. For example, OLEDs can be used in information displays and general lighting applications.
A photovoltaic (PV) device absorbs light and generates electricity. The absorption of light and separation of charges happen in the active materials in a PV device. Organic materials such as conjugated polymers and small molecules can be used as the active materials in PV devices. Organic material based PV devices offer a potentially cheaper alternative over the traditional silicon based photovoltaic devices, such as solar cells and photodetectors.
A simple OLED comprises electroluminescent or light emitting organic material(s) sandwiched between two electrodes (J. H. Burroughes et al, Nature 347, 539 (1990)), one of which (frequently anode) is transparent to allow light to be extracted from the device and used for display or lighting. When the device is connected to an external voltage/current source, holes are injected from the anode and electrons injected from the cathode into the light emitting layer. The holes and electrons then migrate towards the opposite electrode under the influence of the applied electric field. In the recombination zone in the organic layer, holes and electrons encounter each other. A fraction of them recombine and form excitons or excited states. Some of the excitons then decay radiatively to the ground state by spontaneous emission and emit light. To improve the device performance, additional layers that can help inject/transport holes/electrons into the organic layer can be added. (C. W. Tang et al., Appl. Phys. Lett. 51, 913 (1987); P. K. H. Ho, et al., Nature 404, 481(1998)).
The multilayer device configuration offers the advantage of being able to optimize the properties of the materials used for each layer, and adjust the layer thickness according to the property of the materials. However, the cost associated with manufacturing increases commensurately with the number of layers. With device design for manufacturability as a guide, a two-layer design becomes the minimum number of layers that provides anode-ion buffering and charge-carrier transport differentiation (M. T. Bernius et al., Adv. Mater. 12, 1737 (2000)). In a double layer device, each layer has multiple functions, e.g. charge injection/transport or charge transport/emission.
For hole injection/transport layer applications, a number of semiconductive materials have been demonstrated in the prior art. Poly(N-vinylcarbazole) (PVK) has been used as hole transport layer in small molecule OLEDs (X. Z. Jiang et al., Synth. Met. 87, 175 (1997)). Aromatic amines have been used as hole transporting layer (C. W. Tang et al., Appl. Phys. Lett. 51, 913 (1987)). A series of triarylamine containing perfluorocyclobutanes (PFCBs) that are in-situ thermally polymerized have been reported as hole injection/transport layer in OLEDs. The highest occupied molecular orbital (HOMO) level of the PFCBs ranges from −5.1 to −5.3 eV, which matches well with the work function of indium tin oxide (ITO), a commonly used anode for LEDs. Once polymerized, the PFCBs are insoluble in most organic solvent, which enables the fabrication of multilayer LEDs (X. Z. Jiang et al., Adv. Funct. Mater. 12, 745 (2002)). Using a high glass transition temperature (Tg) hole transport polymer with triphenyldiamine as the side chain, an OLED with a luminous efficiency of 20 Im/W and an external quantum efficiency of 4.6% at 14 cd/m2 has been achieved. The device quantum efficiency can be increased by tuning the ionization potential of the hole transport moieties (G. E. Jabbour et al. IEEE Journal of Quantum Electronics 36 (1), 12 (2000)).
Conducting polymers have also been utilized as hole injection/transport material in OLEDs. Yang et al. disclosed the use of polyaniline (PANI) or a combination of PANI and ITO as the transparent anode of a polymer LED with poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV), as the active layer (Y. Yang et al., Appl. Phys. Lett. 64, 1245 (1994)). Poly(3,4-ethylenedioxythiophene) (PEDOT) has been used to facilitate hole injection/transport (U.S. Pat. No. 6,391,481). Higgins et al disclosed an emeraldine base PANI protonated with polystyrene sulfonic acid as hole transport layer (R. W. T. Higgins et al., Adv. Funct. Mater. 11(6), 407 (2001)).
OLEDs represent a promising technology for large, flexible, lightweight, flat-panel displays. However, the OLED devices need further improvement for practical applications. Similarly, the performance of organic photovoltaic devices needs further enhancement for practical applications.